1. Field of the Invention
The present invention relates to a recording method in an inkjet recording apparatus, and more particularly to a recording method of forming a picture image on a recording medium having a relatively low absorption ability of ink such as an original picture of an overhead projector (OHP).
2. Description of the Related Art
As is well known, in an inkjet recording apparatus, the recording quality is greatly affected by the physical properties of a recording medium. In general, a specialized paper such as an inkjet printer single-purpose paper is recommended for each recording apparatus. The specialized paper is specified in its physical properties such as the diffusion rate, absorption rate and reflection density of the printed ink droplet. At present, the inkjet recording apparatus is designed so that the desired printing quality can be obtained with use of such a specialized paper.
When the recording is performed, using such an inkjet recording apparatus, on a recording paper other than the specialized paper, particularly, a translucent recording medium (hereinafter, referred to as OHP film) used for an overhead projector and the like which has a relatively low absorption ability of ink, a dot diameter is made smaller as compared with that for the specialized paper depending on the difference between the physical properties of the recording mediums.
FIG. 4 is a view showing printed dots on a recording medium. In the normal printing state by use of the specialized paper or the like, as shown in FIG. 4A, the printing is performed with a dot diameter larger than a dot interval D calculated on the basis of 1/resolution; and in the ideal state, with a dot diameter d expressed by the following equation: ##EQU1##
With such a dot diameter, even when the printing is performed over the whole surface, the portions where the under color of the recording medium appear are made smaller, thus making it possible to obtain an excellent printing quality. However, for the recording on the OHP film or the like, as shown in FIG. 4B, a dot diameter d' is smaller than the dot interval D, and a large gap is generated between a dot and the surrounding dots. When the printing is performed over the whole surface in such a state, there occur a lot of non-printed portions due to the gaps. This causes such a disadvantage that the printing density is lowered and thus the printing quality is significantly degraded. In addition, due to the contraction of the dot, the ink is not easily absorbed and dried, that is, not easily fixed to the surface of the OHP film or the like.
To be printed with a dot diameter d larger than the dot interval D as shown in FIG. 4A, the OHP film has been improved in the diffusion and fixing performances of the ink droplet by applying a surface treatment thereon. However, the OHP film having the same characteristics as the specialized paper is not obtained by improvement of the diffusion and fixing performances, and is disadvantageous as yet in differing from the specialized paper in the absorption rate of ink. Namely, in contrast to the specialized paper, the OHP film has a small absorption rate of ink. Consequently, in the printing with a large dot diameter d, as shown in FIG. 4c, the adjacent ink droplets are contacted with each other, and the original shapes of the ink droplets are deformed. This is called a beading phenomenon.
FIGS. 5(A) and 5(B) are views showing the densities of ink droplets on a recording medium In this figure, a', b', c' indicate ink droplets, and a, b, c indicate intervals. FIG. 5A is for the printing performed on an ideal recording medium, wherein each ink droplet is slightly overlapped in the peripheral portion, being permeated in a recording medium, and is fixed with an uniform density. However, when a beading phenomenon is generated, for example, when the ink droplet a', is contacted and bonded with the ink droplet b , the dot shapes of the ink droplets are deformed. Thus, the area b being high in density is formed at the bonded portion in the area a of the bonded droplets, whereas the area c being low in density is formed around the bonded ink droplets. Namely, due to the generation of the beading phenomenon, the printing density is not made uniform just as in FIG. 5A, resulting in the nonuniformity in density. As for the nonuniformity in density, when color printing is performed, there occurs a beading phenomenon between different colors. This brings about a disadvantage in causing the bleeding between the picture elements. In addition, the OHP film is placed on an overhead projector, and is largely projected by transmission of a light. In the enlarged projection of the OHP film, the nonuniformity in density and the bleeding are also enlarged, which causes a disadvantage in significantly degrading the quality of the projected image.
Conventionally, various methods have been developed for performing the printing on a recording paper having a relatively low absorption ability of ink such as the OHP film. For example, in a technique disclosed in Unexamined Japanese Patent Publication No. HEI 2-4523, the printing for one line is repeated for each color by the number of colors, to prevent the generation of bleeding. However, for one color, this technique performs the printing of adjacent dots. In other words, the beading phenomenon as described above is not under consideration. Accordingly, for a recording apparatus in which the dot diameter is smaller than the dot interval, or excellent picture can be formed; however, for the recording apparatus in which the dot diameter is larger than the dot interval, it is impossible to prevent the nonuniformity in density due to the beading phenomenon in which the adjacent ink droplets are contacted with each other and the original shapes of the ink droplets are deformed. As a result, there remains the generation of the bleeding due to the color deviation caused by the nonuniformity in density for each color, which cannot be prevented by this process.
Unexamined Japanese Patent Publication No. SHO 63-312155 has disclosed a such technique that gaps between dots are buried by alternately performing two scanning operations, and a time difference is set between the first and second scanning operations, to prevent the bonding of ink droplets. Further, a technique for preventing a beading phenomenon has been disclosed in Unexamined Japanese Patent Publication No. SHO 64-67348, wherein the printing is performed twice with a checker board pattern. In these techniques, particularly, in the latter technique, dots adjacent to each other in the diagonal direction are printed by the first scanning operation; and dots adjacent to each other in the top and bottom and in the right and left directions, which tends to cause a beading phenomenon, are printed by a different scanning operation, to avoid the beading phenomenon. However, in the recording medium such as the OHP film, until the second scanning operation is performed, the fixing of dots formed in the first scanning operation is not completed. As a result, a beading phenomenon is generated between the dots formed in the second and the first scanning operations, to cause the nonuniformity in density and the bleeding, thus degrading the printing quality.